OAuth2 Developer Keys

I've developed a Google Sheets Addon to upload quiz questions to a Canvas quiz. I have OAuth2 login working for my test account, but I can't find any documentation about how to create a public key/id pair for other institutions to use the app.

So, if I create an id/key pair under my domain's account, are those credentials valid for another domain using the app (in other words, I hard code those into the source)? Or, will those developer keys need to be provided for each domain using the app?

Is there a way to register a Canvas third-party app outside the LTI protocol?